For example, position-measuring devices of this kind may be used for a wafer-inspection machine in which an optical, electron-optical or ion-optical microscope must be positioned very precisely over a silicon wafer. The wafer represents the object, the microscope is the tool. Instead of microscopes, however, any other sensors and processing tools may be used as a tool. Quite generally, the range of application includes any extremely precise, (at least) two-dimensional positioning.
For the high-precision measurement of one-dimensional positions, generally position-measuring devices or encoders are used which optically scan a grating scale, and in so doing, generate incremental sinusoidal signals and cosinusoidal signals. Such signals are interpolated in the evaluation electronics, so that a very small measuring step results, which only now amounts to a small fraction of the signal period. In this context, measuring steps of fractions of a nanometer are completely feasible.
In the case of a two-dimensional position determination in the X-direction and Y-direction, often two one-dimensional sliding units are used which are disposed perpendicularly relative to each other, and which in each case are equipped with a one-dimensional encoder. In this context, until now it has not been possible to comply with the “Abbe principle”, so that measuring errors result due to tilt errors in the linear guides. The Abbe principle requires that the respective measuring system be aligned coaxially with respect to the shift direction in which measurements are to be made. As a rule, the Abbe distances, that is, the distances between the measuring axes of the encoders and the center of the tool, hereinafter referred to as tool center point or TCP, are considerable. According to the definition used here, the measuring axis of an encoder extends through the effective measuring point of the encoder along a measuring direction. However, the straightness deviations of the linear guides also lead to measuring errors which are not detected by the encoders. Both error influences are only partially reproducible, so that even a machine calibration often cannot furnish the precision required. Typical non-reproducibility because of guide deviations are in the range of 100 nm.
For these reasons, plane-mirror laser interferometers are used today for high-precision, two-dimensional position determinations. The measuring axes of two plane-mirror laser interferometers perpendicular to each other are arranged in conformance with the Abbe condition such that they intersect in the fixed TCP. In this manner, all guide deviations of the XY-table are detected, and therefore no longer have any appreciable influence on the accuracy. However, laser interferometers operated in air have the disadvantage that fluctuations in the refractive index of the air lead to substantial measuring errors. Thus, given an air gap of 30 cm, even under favorable laboratory conditions, one must expect errors of 20 nm to 50 nm. This is not sufficient for the rising number of applications having an accuracy requirement of 1-10 nm.
German Patent No. 25 21 618 describes a configuration of two crossed scales, which are optically scanned at the point of intersection. The one scale bears a “longitudinal graduation”, that is, a grating having grating lines parallel to the narrow edge of the scale surface. The second scale is provided with a transverse graduation, that is, the grating lines extend parallel to the long edge of the scale surface. Due to the perpendicular arrangement of the longitudinal graduation and transverse graduation, the grating lines are parallel to each other, so that the relative position may be recorded at the point of intersection using suitable scanning optics.
To record position in conformance with the Abbe condition, the longitudinal graduation in the XY-plane is aligned with respect to the TCP and, like the TCP, is fixed. The transverse graduation is secured to the XY-table. The second measuring direction includes a second perpendicularly disposed pair of longitudinal graduation and transverse graduation, whose longitudinal graduation in the XY-plane is likewise oriented to the TCP. The two scanning optical systems must each be shifted along the longitudinal graduations, so as constantly to be able to scan at the points of intersection. However, German Patent No. 25 21 618 describes no suitable scanning optics, nor does it give any instructions as to how the scanning optics are to be guided along the longitudinal graduation. Upon more careful observation, the Abbe distances are brought to zero only in the XY-plane. However, Abbe distances unequal to zero remain in the Z-direction, since the graduations (reference numerals 3 and 8 in FIG. 2 of German Patent No. 25 21 618) and the object (reference numeral 6 in FIG. 2 of German Patent No. 25 21 618) are disposed in different Z-positions. Tiltings of the table about the X-axis or Y-axis therefore continue to lead to measuring errors.
Further details regarding a measuring system of this kind are described in European Published Patent Application No. 1 734 394. For example, specific arrangements are described for the necessary guideways of the scanning heads along the longitudinal graduations. No suitable scanning optics are described. The Abbe distances in the Z-direction are not taken into consideration, as is evident from FIG. 3 of European Published Patent Application No. 1 734 394.
PCT International Published Patent Application No. WO 2007/034379 describes various scanning optics for position-measuring devices of this nature, in order to scan the longitudinal and transverse graduations in such configurations. The Abbe distance in the Z-direction is neither mentioned nor considered or minimized, either. Upon more careful analysis, the scanning optics described are hardly suitable for the measuring task, since the effective measuring point of the scanning optics, hereinafter referred to as the neutral pivot (NP) of the encoder, cannot be leveled with the object in the Z-direction. A method for analyzing the position of the neutral pivot is described further below, and leads to the following results. The neutral pivot of the scanning optics according to FIG. 5 or 6 of PCT International Published Patent Application No. WO 2007/034379 lies far above the grating having reference numeral 5, approximately at the distance of the first grating having reference numeral 5 from the grating having reference numeral 4. When scanning according to the configurations described (FIG. 3 of PCT International Published Patent Application No. WO 2007/034379), a considerable Abbe distance therefore inevitably results in the Z-direction. The scanning optics according to FIG. 8 of PCT International Published Patent Application No. WO 2007/034379 have a neutral pivot at the height of the grating having reference numeral 4. Since the mirror having reference numeral 7 in the Z-direction projects beyond the grating plane, which then optimally would have to be the object plane, this configuration is not compatible with the normally small distance between tool and object, since the tool and the mirror having reference numeral 7 stand in the way of each other. Something similar holds true for the scanning optics according to FIGS. 9 and 10 in PCT International Published Patent Application No. WO 2007/034379, as well. The neutral pivot of the scanning optics according to FIGS. 11 and 12 of PCT International Published Patent Application No. WO 2007/034379 lies far below the grating having reference numeral 4, approximately at the distance of the grating having reference numeral 4 from the grating having reference numeral 5. A leveling of the Z-position of the neutral pivot with the object is impossible here, as well.
European Published Patent Application No. 1 734 394 and PCT International Published Patent Application No. WO 2007/034379 exclusively describe scanning optics for position-measuring devices of the kind which are disposed and moved above the stationary scale having the longitudinal graduation.
European Published Patent Application No. 1 837 630 describes further scanning optics for scanning longitudinal and transverse graduations. In the various specific described therein, there is no information concerning the suitable Z-position of the neutral pivots and their position relative to the object to be measured. An additional measurement in the Z-direction at least three positions of the table is described, so that all six degrees of freedom of the table are included. The tilts Rx and Ry about the main axes of motion X and Y of the table can therefore be corrected by suitable digital signal processing. However, this requires considerable extra expenditure for the additional measurements of Rx and Ry, which is only justified in the case of very costly machines such as wafer steppers. The additional measurement of the Rx-tilt and Ry-tilt is necessary, since these tilts must be changed in operation by suitable actuators. For most other applications such as, for example, for wafer-inspection machines, no Rx- and Ry-actuators are used, so that basically, one could dispense with their measurement. However, the less than ideal position of the neutral pivots in the direction of the Z-axis remains again in this case.
German Patent No. 25 21 618, European Published Patent Application No. 1 734 394, PCT Published Patent Application No. WO 2007/034379 and European Published Patent Application No. 1 837 630 discussed above each confine themselves to machine designs having a fixed TCP. They give no information as to how other machine designs having a moving TCP can be improved by crossed longitudinal and transverse graduations.
Furthermore, the interpolation accuracy of the position-measuring devices or encoders must be considered. Since in the case of machines, axis directions X and Y are often at the same time the main movement directions as well, the transverse graduations are moved along the grating lines. Signal interferences due to grating tolerances lead to interpolation errors which cannot be offset by electronic compensation methods, since for an adequate error correction, these compensation methods always need a phase shift of the signals, and therefore a movement component in the measuring direction, as well.